As a conventional technique for correcting tones of an image, the “histogram equalization method” of calculating a luminance value or density value of each pixel across an entire image area of an input image, creating a histogram showing the number of occurrences of pixels having the same tone value or pixels included in the same section obtained by division into less number of sections than a quantized number of tones, and carrying out tone correction processing across the entire image area such that the created histogram has an optimum shape is widely well known (cf. non-patent document 1). With this histogram equalization method of correcting tones of an image using the histogram, optimizing a tone distribution in the case where tones in the image area concentrate in a certain level can achieve improved tone characteristics of the whole image area.
Further, techniques applying such “histogram equalization method” to backlight correction are also proposed (cf. patent documents 1 and 2). These techniques determine the backlight condition on the basis of a histogram of the whole image area to carry out tone correction so as to approach an optimum histogram.
On the other hand, another technique using “local contrast correction method” of diving an image area into a plurality of blocks and correcting a local contrast which is a local contrast on the basis of an average luminance value in a divided block, not the whole image area, is proposed (cf. patent document 3). The use of this “local contrast correction method” as well as the use of the Craik O'Brien effect of giving a luminance gradient to a contour to cause visual illusion because of human eye characteristics can artificially increase the dynamic range.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-69825
Patent document 2: Japanese Patent Application Laid-Open No. 2003-299107
Patent document 3: Japanese Patent Application Laid-Open No. 9-65252
Non-patent document 1: written by ANIL K. JAIN, “Fundamentals of Digital Image Processing”, Prentice-Hall International, Inc. publishing, 1989, pp. 241-244
However, in the case of applying the aforementioned conventional technique for correcting tones of an image, the “histogram equalization method” or “local contrast correction method”, directly to backlight correction, a problem of imposing a limit in improving the local contrast in a bright region or a dark region arises. The reason for this issue will be described below in detail.
That is, the conventional technique adopting the “histogram equalization method” uses the same tone correction curve across an entire image area, which causes no change in sequence of tone levels in one image area unless the tone correction curve has a negative inclination or unless a discontinuous tone correction curve is used. For instance, in the case where a shadow region and a highlight region concentrate in the vicinity of certain tones, respectively, as shown in a histogram of an input image in FIG. 10, tone correction using the “histogram equalization method” results in a distribution as indicated by solid lines shown in FIG. 11. That is, a histogram 601 of a shadow region is converted into a wider tone distribution 611, which increases the local contrast in the shadow region. A histogram 602 of a highlight region is similarly converted into a wider tone distribution 612, which increases a local contrast of the highlight region. However, neither of the aforementioned conventional techniques allows such conversion as shown in FIG. 12 that a histogram 621 of a shadow region and a histogram 622 of a highlight region after conversion are reversed in sequence of tone levels in some tones. Therefore, since the sequence of tone levels is held after correction in the case where a dark part of a bright region in one image area is brighter than a bright part of another dark region in the same image area, the dark part of the bright region in one image area becomes brighter than the bright part of the dark region in the same image area or is rounded to become identical. Therefore, there exists a limit in correction amount of a dark part of a bright region and a bright part of a dark region, which inevitably causes a limit in improving the local contrast in a bright region in an image area or a dark region in an image area.
Further, the conventional technique adopting the “local contrast correction method” serves to maintain the average luminance value of a local region to some degree in order to perform local contrast correction throughout the entire tones while there may be a phenomenon in which tone levels are reversed in sequence according to places. Therefore, there is a limit on the backlight correction effect for a bright region or a dark region as a whole. For instance, in the case where a bright region or a dark region continues over an area of the size of a divided block or an area exceeding the size of predetermined times larger than a divided block determined by the filter shape per block, tone levels are maintained after correction with respect to the average luminance level of the continuing bright region or dark region as a whole, and the average luminance level after correction does not vary greatly as compared to that prior to the correction. Thus, the contrast of a local region is improved, however, there is a limit on the backlight correction effect for a bright region or a dark region as a whole.
Furthermore, since the conventional technique adopting the “local contrast correction method” performs local contrast correction throughout the entire tones, applying that correction method to backlight correction as-is also increases the local contrast of a block in a midtone level not of concern per se when trying to improve the correction effect on a dark region or a bright region. Therefore, the local contrast can only be improved to such degree that the midtone level is not corrected excessively.